This invention relates to a semiconductor laser device, a semiconductor laser element with at least one exit surface from which laser light can emerge, which in a first direction has greater divergence than in the second direction which is perpendicular to it, at least one reflection means which is located spaced apart from the exit surface outside of the semiconductor laser element, with a reflecting surface which can reflect back at least parts of the light which has emerged from the semiconductor laser element through the exit surface into the semiconductor laser element, such that the mode spectrum of the semiconductor laser element is influenced thereby, and a lens means which is located between the reflection means and the semiconductor laser element and which can at least partially reduce the divergence of the laser light at least in the first direction.
A semiconductor laser device of the aforementioned type is known from OPTICS LETTERS, 2002, Vol. 27, No. 3, page 167 to 169. In the semiconductor laser device described in it, a laser diode which is made as a so-called broad strip emitter is used as a semiconductor laser element. In these broad strip emitters, for example, there are exit surfaces for the laser light which have a width of roughly 100 microns and a height of roughly 1 micron. Over this width, within the internal resonator which is formed by the end surfaces of the laser diode, an entire series of different transverse modes of the laser light can be formed. At the same time, an entire series of longitudinal modes, i.e. different wavelengths of laser light, can also occur. In particular, the numerous different transverse modes adversely affect the beam quality of the laser beam emerging from this broad strip emitter. This laser radiation cannot be optimally focused. The longitudinal modes lead to spectral broadening which is not desirable for various applications.
In the aforementioned publication, an external resonator is proposed which has a highly reflecting planar mirror. Between the planar mirror and the exit surface of the semiconductor laser element, facing the external resonator on the one hand, there is a fast axis collimation lens and on the other hand, between the fast axis collimation lens and the planar mirror, there is a spherical convex lens. The fast axis collimation lens is used to collimate the light of the broad strip emitter which is much more dramatically divergent in the first direction. The spherical lens is used to focus the light which has been reflected back by the planar mirror such that it is imaged essentially back onto the exit surface. Furthermore, in the external resonator there is an aperture diaphragm. Both the aperture diaphragm and also the planar mirror are located outside the optical axis of the external resonator and outside of the normal or middle perpendicular on the exit surface. It is shown that for broad strip emitters the stronger modes generally emerge at a small angle to the normal on the exit surface from the latter. Through the aperture diaphragm which is positioned outside of the axis thus only portions of such a mode which emerges from the exit surface at an angle will be incident on the mirror and will be reflected by it through the aperture diaphragm and the spherical lens back onto the exit surface. Only light from one or more such modes is thus reflected back through the exit surface into the laser diode. In this way the laser diode can be caused to stimulate oscillations essentially on this mode so that the mode spectrum of the semiconductor laser element is reduced essentially to the transverse mode.
The laser light is decoupled from the external resonator, according to the aforementioned publication, by the component beams of the corresponding transverse mode which emerge from the exit surface at the oppositely identical angle being able to emerge from the semiconductor laser device past the aperture diaphragm and the planar mirror.
The disadvantage in the aforementioned arrangement is that there are a comparatively large number of different optical elements in the external resonator. Besides the fast axis collimation lens, they are the spherical lens, the aperture diaphragm, and the planar end mirror. Due to the many different optical elements which are provided in the external resonator, on the one hand imaging errors occur increasingly and on the other hand, large losses arise since these elements are located within the laser resonator. But in this way the attainable output power of such a semiconductor laser device is greatly limited. At the same time the output powers which can be achieved with such a semiconductor laser device can only be attained with high cost. In addition such a semiconductor laser device can only be calibrated with difficulty.
According to the existing art, an attempt is made to influence the mode spectrum of the semiconductor laser elements by structuring the active zone of the semiconductor laser element. This structuring can include changes of the refractive index in different directions, so that propagation of individual preferred transverse laser modes is preferred by these refractive indices which change in different directions. Furthermore it is possible, for example, by different degrees of doping, to act on the number of electron-hole pairs available for recombination so that at different locations of the active zone different amplifications of the laser light are possible. The two aforementioned methods for giving preference to individual transverse modes are associated with consideration production cost and likewise do not yield actually satisfactory beam quality or output power of the semiconductor laser device.
It is an object of this invention to devise a semiconductor laser which ensures comparatively high beam quality and high output power with simple means.
This is achieved as claimed in the invention by the features according to claims 1 or 6.